Scanner with reed relays

ABSTRACT

Electrical switching apparatus such as a scanner comprises a plurality of reed relays, each of which has an operating coil, and at least one reed contact unit which is mounted outside the coil, to reduce thermal E.M.F. generation. The reed contact units are mounted on a first printed circuit board, and control respective analogue signals passing through the printed circuit tracks of this first board, while the coils are mounted on, and respectively energised via the printed circuit tracks of, a second printed circuit board parallel to the first board. In this way, interference between the scanned analogue signals and the relay energising signals is reduced.

United States Patent 1191 Parmenter l- SCANNER WITH REED RELAYS [75] Inventor: George Shearer Parmenter, Hartley Wintney, England [73] Assignee: The Solartron Electronic Group Limited, Farnborough, Hampshire, England [22] Filed: June 19, 1972 [21] Appl. No.: 264,038

[30] Foreign Application Priority Data June 19. 1971 Great Britain 28891/71 Dec. 28, 1971 Great Britain 60262/71 [52] US. Cl 335/151, 335/152 [51] Int. Cl H01h 1/66, HOlh 51/00 [58] Field of Search 335/112, 151, 152, 153, 335/154 [56] References Cited UNITED STATES PATENTS 3.403602 10/1968 McWhorter 335/152 X 3.582.844 6/1971 Launt et a1. 335/154 2.985.733 5/1961 Kamps 335/151 Primary Examiner-David Smith, J r. I V Attorney, Agent, or Firm-Steward F. Moore, William 'R. Sherman, Roylance, Abrams, Berdo & Kaul AES T iiKCT Electrical switching apparatus such as a scanner comprises a plurality of reed relays, each of which has an operating coil, and at least one reed contact unit which is mounted outside the coil, to reduce thermal E.M.F. generation. The reed contact units are mounted on a first printed circuit board, and control respective analogue signals passing through the printed circuit tracks of this first board, while the coils are mounted on, and respectively energised via the printed circuit tracks of, a second printed circuit board parallel to the first board. In this way, interference between the scanned analogue signals and the relay energising signals is reduced.

7 Claims, 4 Drawing Figures SCANNER WITH REED RELAYS This invention relates to reed relays, and to electrical switching apparatus, such as scanners, employing reed relays.

In a known form of reed relay, the reeds comprise two elongated flexible electrical contacts of magnetic material, which are sealed within a glass capsule and are so secured at the opposite ends of the capsule that respective free end portions of the reeds lie alongside one another in spaced-apart relationship: the assembly of the reeds and the capsule will hereinafter be referred to as a reed contact unit. The reed contact unit is mounted within an electrical coil, with the reeds substantially parallel to the axis of the coil, so that the passage of an electric current through the coil causes the reeds to attract one another, thereby establishing electrical contact between them.

The passage of an electric current through the coil also generates heat which, in this known form of reed relay, is transmitted to the reeds. It has been found that, in operation, differences between the respective temperatures of the reeds arise, particularly where there is asymmetry in the arrangement of the reeds within the coil: typically, where the coil is dissipating three quarters of a watt, a temperature difference between the reeds of about one Centigrade degree may arise. This temperature difference may be transferred to the respective junctions between the reeds and the circuitry of which they form part, which can, in turn, lead to the production of a thermal E.M.F. of up to 40 microvolts between the reeds. This is particularly disadvantageous, for example, when the reed relay forms part of a scanner, and is being used to switch low voltages which are to be measured in an electrical measuring instrument such as a digital voltmeter.

Moreover, in electrical switching apparatus such as scanners, employing reed relays for selectably or sequentially connecting each one of a plurality of analogue input signals to an output, e.g. for measurement purposes, it is desirable that the analogue input signals be isolated from the signals used to energise the relays, so as to reduce the possibility of interference between the relay energising signals and the analogue signals. At the same time, it is desirable to use a printed circuit type of construction, for ease and economy of manufacture and for good accessibility and replaceability.

it is an object of the present invention to provide a reed relay in which differential heating of the reeds by the coil is substantially reduced, and which is relatively simple and economical to manufacture. It is a further object of the present invention to provide electrical switching apparatus, such as a scanner, employing reed relays, in which interference between the relay energising signals and the signals being switched by the relays is substantially reduced.

According to one aspect of the present invention, therefore, a reed relay comprises a coil, two plate-like pole pieces which are respectively positioned at opposit e ends of the coil and which are substantially perpendicular to the axis thereof, each pole piece having a respective first bent-over portion which extends axially into the coil and a respective second bent-over portion which extends substantially parallel to the axis of the coil and towards the second bent-over portion of the other pole piece, and at least one reed contact unit which is positioned outside the coil and is adapted to be mounted parallel to and adjacent a line joining the second bent-over portions, the first bent-over portions being arranged to overlap each other within the coil, and together be a push fit therein.

In a preferred embodiment of this aspect of the invention, the pole pieces are substantially H-shaped in plan view, the first and second bent-over portions extending substantially perpendicularly to the plane of the H-shape, both in the same direction, from the opposite sidcs of the cross-piece of the H-shape.

The pole pieces may conveniently be made by suitably bending a blank stamped from a mild steel sheet.

According to another aspect of the present invention, a reed relay comprises a core member, a coil wound upon the core member two pole pieces which are respectively positioned at opposite ends of thecoil and which are substantially perpendicular to the axis thereof, the pole-pieces and the core member being integrally formed from a plastics material loaded or filled with a ferromagnetic material, and at least one reed contact unit which is positioned outside the coil and is adapted to be mounted parallel to the axis of the coil and adjacent a line extending perpendicularly between the pole pieces.

The ferromagnetic material may be iron powder, and the pole pieces and the core member may be formed by moulding.

The invention also includes electrical switching apparatus such as a scanner, or a digital voltmeter or other electrical measuring instrument, incorporating at least one reed relay in accordance with any of the preceding statements of invention.

According to a further aspect of the present invention, electrical switching apparatus such as a scanner includes:

a plurality of reed relays each comprising a coil, at least one reed contact u'nit positioned outside the coil, and magnetic circuit means for transmitting magnetic flux produced by the coil to the reed contact unit to as to cause, in use, electrical contact between the reeds thereof;

at least one first printed circuit board having a plurality of first electrically conductive tracks thereon for the passage of respective analogue electrical signals, each reed contact unit being mounted on the first board and connected in series with a respective one of the first tracks; and

at least one second printed circuit board which is substantially parallel to the first board and has a plurality of second electrically conductive tracks thereon for the passage of respective relay energising signals, each of the coils being connected in series with a respective one of the second tracks;

the reed relays being positioned between the first and second boards.

Each coil may conveniently be mounted on the second board.

Preferably, the first and second boards are secured to each other in spaced-apart relationship.

For each reed relay, magnetic circuit means may comprise two pole-pieces respectively positioned at opposite ends of the coil, and the pole pieces may conventiently also serve to space-apart the first and second boards.

In a preferred embodiment of this aspect of the invention, for each reed relay, the axis of the coil is parallel to the plane of the second board, and the pole pieces comprise respective plate-like each having a respective first bent-over portion which extends axially into and is secured within the coil and a respective second bentover portion which extends substantially parallel to the axis of vthe coil and towards the second bent-over portion of the other pole piece, the reed contact unit being positioned parallel to and adjacent a line joining the second bent-over portions.

The invention will now be further described, by way of non-limitative example only, with reference to the accompanying drawings, of which:

FIG. 1 is a cross-sectional view of part of a scanner in accordance with the present invention, incorporating a reed relay also in accordance with the present invention;

FIG. 2 is a plan view of a pole piece for use in the reed relay of FIG. I;

FIG. 3 is a greatly simplified schematic circuit diagram of the scanner of FIG. 1;

and FIG. 4 is an alternative construction for the polepiece of FIG. 2.

Referring first to FIG. 1, the reed relay comprises an electrical coil wound on a hollow plastics bobbin 12. The hollow interior of the bobbin I2 is substantially rectangular in cross-section and the bobbin is provided I pole pieces 16, as viewed in FIG. 2, are provided with respective reduced-width portions 20, which may conveniently project through holes in a printed circuit board 21 of the scanner, so as to facilitate mounting the pole pieces I6 on the board. The upper limbs 22 of each of the pole pieces 16 are substantially uniform in width, and are shorter than the lower limbs 18.

The H-shape of each of the pole pieces 16 has a respective cross-piece 24. A bent-over portion 26 extends perpendicularly to the plane of the H-shape of one of the pole pieces 16, from the side of the cross- I piece 24 adjacent the lower limbs 18, into the hollow interior of the bobbin l2, and is almost equal in length to the axial length of the bobbin. A corrsponding bentover portion of the other pole piece 16 is indicated at 26a and is similar to the portion 26 but slightly displaced with respect thereto, so that the portions 26, 26a overlap eachother within the hollow interior of the bobbin l2 and together are a push-fit therein. A respective further bent-over portion 28 also extends perpendicularly from the plane of the H-shape of each of the pole pieces 16, in the same respective directions as the portions 26, 26a, but from the side of the cross-piece 24 adjacent the upper limbs 22. The portions 28 are substantially aligned with each other in a common plane parallel to but displaced from the common axis of the coil 10 and bobbin l2, and are shorter than the portions 26, 26a, so that their adjacent ends are spaced apart.

Each of a plurality of reed contact units 30 comprises two elongated reeds 32 sealed in a substantially cylindrical glass capsule 34 and is mounted as will hereinafter be described outside the coil 10. The reeds 32 are so secured at opposite ends of the capsule 34 that they are substantially parallel, with their free end portions 36 overlapping and slightly spaced apart. They are made from a nickel alloy, and the portions 36 preferably have a diffused gold coating to provide good electrical contact properties.

Typically there may be four reed contact units 30, and they are arranged substantially parallel to each other and to the common axis of the coil 10 and bobbin 12, and slightly displaced from the common plane containing the bent-over portions 28 on the side of this common plane remote from the coil 10 and bobbin 12. The reed contact units 30 thus extend from the region between the upper limbs 22 and one of the pole pieces 16 to the region between the upper limbs 22 of the other pole piece 16.

As already mentioned, the pole pieces 16 are mounted on the printed circuit board 21, and it will be appreciated that the coil 10 and bobbin 12 are secured to the board 21 by means of the bent-over portions, 26, 26a of the pole pieces. In' this case, the reed contact units 30 can conveniently be mounted on a further printed circuit board 38 of the scanner, which further board is parallel to, and fixedly mounted on, the board 21. As can be seen in FIG. 1, the pole pieces 16 also serve as spacers to space-apart the boards 21, 38.

In operation, when an electric current is passed through the coil 10 a magnetic field is produced along the axis of the coil, which magnetic field is entrained by the pole pieces 16 to produce a magnetic field extending between the bent-over portions 28. This field is parallel to the reeds 32 and causes them to attract one another, thereby establishing electrical contact between their end portions 36. The upper limbs 22 of the pole pieces 16 serve to reduce magnetic flux leakage and concentrate the magnetic flux in the region between the bentover portions 28. Since the reed contact units 30 are mounted outside the coil I0, heating of the reeds 32 by the coil 10 in operation is substantially reduced. The reed relay hereinbefore described is thus particularly suitable for use in switching apparatus such as scanners, or in other applications where it is desired to switch a low voltage which is to be measured in an electrical measuring instrument such as a digital voltmeter.

Each of the pole pieces 16 is produced by stamping a suitably shaped blank from a mild steel sheet, and then appropriately bending or folding the blank to produce the bent-over portions 26, 26a and 28. It will be appreciated that this is a relatively simple and economical process. v

' The circuit of the scanner is shown in FIG.'3, and comprises a plurality of analogue inputs 40 forming part of an edge connector 42 of the printed circuit board 38. Each of the inputs 40 may be connected to receive an analogue signal from a respective one of a plurality of separate, remotely positioned transducers (not shown), and is connected, via a respective printed circuit track 44 provided on the board 38, to one of the reeds 32 of a respective reed contact unit 30. The other reed 32 of each of the reed contact units 30 is connected to a common printed circuit track 46, which is in turn connected to an output 48 in the edge connected 42. The output 48, which constitutes the analogue output of the scanner, is shown connected to the inputof a digital voltmeter 50, which may for example form part of a data logger and be provided with printout facilities.

The respective coils associated with the reed contact units 30 are shown mounted on the printed circuit board 21, and are connected to be sequentially energised, via respective printed circuit tracks 52 provided on the board 21, by a controller/timer circuit 54 also mounted on the board 21. The circuit 54 may typically sequentially energise the coils 10 for one second each at one second intervals, and repeat the sequence every hour: the timing will be determined in each instance by the measuring and print-out time required by the digital voltmeter 50, and the frequency with which it is desired to take measurements with the transducers.

The printed circuit tracks 52 are also connected, via an edge connector 56, to the outputs 58 of a selector unit 60. The selector unit 60 is manuallyoperable, e.g. by means of numbered push-buttons (not shown), to permit any one of the coils 10 to be energised, so that measurements can be taken at times other than those determined by the controller/timer circuit 54 if desired.

It will be appreciated that the tracks 44, 46 are isolated from the tracks 52 by virtue of being on separate printed circuit boards, 38 and 21 respectively. This substantially reduces the possibility of interference between the relay energising signals from the controller/- timer circuit 54 (or selector unit 60) and the analogue signals from the transducers.

Several modifications may be made to the described embodiments of the invention. For example, the pole pieces 16 need not be H-shaped: in particular, the upper limbs 22 may be omitted. Additionally, the polepieces 16 could be replaced by the pole-pieces 66 shown in FIG. 4. The pole-pieces 66 are substantially parallel, and are joined together by a core member 68 which is formed integrally, e.g. by moulding, with the pole-pieces 66 from a plastics material filled or loaded with a ferromagnetic material such as iron powder. The coil 10 is simply wound upon the core-member 68. Moreover differential heating of the respective junctions between the reeds 32 and the tracks 44 and 46 can be still further reduced by shielding these junctions from air currents by means of moulded shielding members secured to the board 38. These shielding members may be made, for example, from expanded polystyrene, and be provided with cavities which locate over and isolate the junction: such a shielding member is shown dotted at 70 in FIG. 1, the cavities being indicated at 72 and the junctions at 74. Finally many other applications of reed relays in accordance with the present invention are possible.

What is claimed is:

l. A scanner unit for selectively connecting one of a plurality of analog electrical signals to be measured to the input of an electrical measuring instrument, the scanner unit comprising a first printed circuit board having a plurality of first electrical conductive tracks thereon for the passage of the analog electrical signals; a second printed circuit board mounted in substantially parallel relationship to said first board and having a plurality of second electrically conductive tracks thereon for the passage of relay energizing signals; a plurality of reed relays each comprising a coiled conductor connected directly in series with one of said second tracks and responsive to a relay energizing signal thereon to produce magnetic flux, and at least one reed contact unit positioned outside of the coiled conductor and responsive to the magnetic flux produced by said coiled conductor to close normally open contacts therein, each of said reed contact units being mounted directly on said first board, the contacts within each said reed contact unit being connected directly in series with one of said first tracks whereby selective energization of one of said second tracks causes completion of a circuit through at least one of said first tracks to the measuring instrument.

2. A scanner unit according to claim 1 and further comprising means for mounting each coiled conductor on the second board.

3. A scanner unit according to claim 1 and further comprising means for securing the first and second boards to one another in spaced apart relationship.

4. A scanner unit according to claim 1 wherein each reed relay further includes magnetic circuit means for transmitting the flux produced by the coiled conductor to the reed contact unit, said magnetic circuit means comprising two pole pieces respectively positioned at opposite axial ends of the coiled conductor.

5. A scanner unit according to claim 4, wherein said pole pieces include spacer means for spacing apart said first and second boards.

6. A scanner unit according to claim 4, wherein, for each reed relay, the axis of said coiled conductor is parallel to the plane of said second board, and said pole pieces comprise respective plate-like members each having a respective first bent-over portion which extends axially into and is secured within the coiled conductor and towards the second bent-over portion which extends substantially parallel to the axis of the coiled conductor and towards the second bent-over portion of the other pole piece, said reed contact unit being positioned parallel to and adjacent a line joining the second bent-over portions.

7. A scanner unit as claimed in claim 4, wherein, for each reed relay, there is provided a core member upon which the coiled conductor is wound with its axis paral lel to the plane of the second board, and wherein pole pieces and the core member are integrally formed from a plastics material loaded or filled with a ferromagnetic material. 

1. A scanner unit for selectively connecting one of a plurality of analog electrical signals to be measured to the input of an electrical measuring instrument, the scanner unit comprising a first printed circuit board having a plurality of first electrical conductive tracks thereon for the passage of the analog electrical signals; a second printed circuit board mounted in substantially parallel relationship to said first board and having a plurality of second electrically conductive tracks thereon for the passage of relay energizing signals; a plurality of reed relays each comprising a coiled conductor connected directly in series with one of said second tracks and responsive to a relay energizing signal thereon to produce magnetic flux, and at least one reed contact unit positioned outside of the coiled conductor and responsive to the magnetic flux produced by said coiled conductor to close normally open contacts therein, each of said reed contact units being mounted directly on said first board, the contacts within each said reed contact unit being connected directly in series with one of said first tracks whereby selective energization of one of said second tracks causes completion of a circuit through at least one of said first tracks to the measuring instrument.
 2. A scanner unit according to claim 1 and further comprising means for mounting each coiled conductor on the second board.
 3. A scanner unit according to claim 1 and further comprising means for securing the first and second boards to one another in spaced apart relationship.
 4. A scanner unit according to claim 1 wherein each reed relay further includes magnetic circuit means for transmitting the flux produced by the coiled conductor to the reed contact unit, said magnetic circuit means comprising two pole pieces respectively positioned at opposite axial ends of the coiled conductor.
 5. A scanner unit according to claim 4, wherein said pole pieces include spacer means for spacing apart said first and second boards.
 6. A scanner unit according to claim 4, wherein, for each reed relay, the axis of said coiled conductor is parallel to the plane of said second board, and said pole pieces comprise respective plate-like members each having a respective first bent-over portion which extends axially into and is secured within the coiled conductor and towards the second bent-over portion which extends substantially parallel to the axis of the coiled conductor and towards the second bent-over portion of the other pole piece, said reed contact unit being positioned parallel to and adjacent a line joining the second bent-over portions.
 7. A scanner unit as claimed in claim 4, wherein, for each reed relay, there is provided a core member upon which the coiled conductor is wound with its axis parallel to the plane of the second board, and wherein pole pieces and the core member are integrally formed from a plastics material loaded or filled with a ferromagnetic material. 